1. Field of the Invention
The present invention relates to a dielectric ceramic composition widely used in high frequency electronic components and, more particularly, to a low-temperature cofired dielectric ceramic composition with a high dielectric constant and a low dielectric loss.
Low-temperature cofired dielectric compositions (materials) means compositions (materials) which can be fired in the temperature range of 800-950xc2x0 C., lower than the melting point of silver (Ag) or copper (Cu), in contrast to the conventional ceramic dielectrics which are sintered at 1,300xc2x0 C. or higher.
2. Description of the Prior Art
In view of the recent trend for miniaturization, lightness and modulization of high frequency electronic devices, it is required to develop dielectric material which can be multi-layered, followed by cofired with the inner electrode.
For multilayering and cofiring with the conventional dielectric materials, it is required to use high melting temperature metals, such as Mo or W, as inner electrode patterns.
Where Mo or W is adopted to form inner electrode patterns, however, an economic disadvantage is incurred because of its high cost. Above all, the skin effect, which causes radio frequency current to concentrate on the surface of a conductor, requires the use of metals with low resistance to reduce the electric loss. Accordingly, the use of metals that are relatively inexpensive, as well as being of high electric conductivity, like Ag or Cu is indispensable.
Therefore, it is very important research project to find a dielectric material which can be sintered at lower temperature than the melting point of Ag (960xc2x0 C.) or Cu (1083xc2x0 C.).
Generally, low-temperature cofired dielectrics can be made by mixing high-temperature sintered material with a small amount of a low melting point material, for example, glass powder or an additive such as CuO, PbO and Bi2O3, V2O5, etc., or by firing glass ceramics comprising ceramics as a filler.
In the case of the latter, the resulting dielectric substrates based on glass show a dielectric constant of 10 or less.
With advantages of low dielectric constants in speeding-up of signal processing and improvement of signal transmission, materials with a low dielectric constant of 10 or less are extensively used for low temperature cofired ceramics (LTCC).
In some cases, meanwhile, substrates made of dielectrics with low dielectric loss and medium dielectric constant (15-100) may be advantageous in terms of circuit design and function without retardation of signal processing, depending on characteristics of applied it circuits.
In addition, the use of dielectrics with high dielectric constants makes the guided wavelength short, leading to a reduction in circuit dimension. Thus, such dielectrics are very useful in applications which give importance to dimensions of electronic devices, as well as having the advantage of reducing insertion loss or frequency deviation in some circuits.
Larger dielectric constants can lower the ratio between the width of transmission lines and the thickness of dielectrics to greater extents, giving circuit designers an opportunity to design better lamination structures.
BaOxe2x80x94Re2Oxe2x80x94TiO2 (Re=rare earth element) dielectric composition with a dielectric constant of 70 or higher can be made low in sintering temperature by the addition of B2O3xe2x80x94SiO2 based glass frit as disclosed in Korean Pat. Laid-Open Publication (No.1999-62997). However, this dielectric composition can be fired at temperatures of 1,000xc2x0 C. or higher which are too high to co-fire Ag electrodes.
Leading to the present invention, the intensive and thorough research into LTCC, conducted by the present inventors, resulted in the finding that in cooperation with glass frit, CuO can serve as a sintering aid to increase the dielectric constant, and play a role in controlling the temperature coefficient of frequency without a large change in Q value.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and provide a dielectric ceramic composition which exhibits high dielectric constant and low dielectric loss and can be sintered at low temperature.
In accordance with an aspect of the present invention, there is provided a dielectric ceramic composition represented by the following chemical formula 1:
Chemical Formula 1
a wt. % {x BaO-y1Nd2O3-y2Sm2O3-w Bi2O3-z TiO2}+
b wt. % (ZnOxe2x80x94B2O3xe2x80x94SiO2xe2x80x94PbO based glass frit)+
xe2x80x83c wt. % CuO
wherein, 10.0 mol %xe2x89xa6xxe2x89xa620.0 mol %; 7.0 mol %xe2x89xa6y1+y2xe2x89xa620.0 mol %; 0.5 mol %xe2x89xa6wxe2x89xa65.0 mol %; 60.0 mol %xe2x89xa6zxe2x89xa680.0 mol %, with the proviso that x+y1+y2+w+z=100; 80.0 wt. %xe2x89xa6axe2x89xa698.0 wt. %; 1.0 wt. %xe2x89xa6bxe2x89xa610.0 wt. %; 1.0 wt. %xe2x89xa6cxe2x89xa610.0 wt. %.
Based on BaOxe2x80x94Nd2O3xe2x80x94Sm2O3xe2x80x94Bi2O3xe2x80x94TiO2, the dielectric ceramic composition of the present invention comprises ZnOxe2x80x94B2O3xe2x80x94SiO2xe2x80x94PbO glass frit and CuO. The composition composed of BaOxe2x80x94Nd2O3xe2x80x94Sm2O3xe2x80x94Bi2O3xe2x80x94TiO2 alone exhibits a sintering temperature of 1,350xc2x0 C. or higher, which is too high to co-fire silver (Ag) electrodes, which melt at 961xc2x0 C. In the present invention, glass frit and CuO are adopted to induce the liquid-phase sintering of the base composition, whereby Ag electrodes can be co-fired.
The driving force for the densification of the liquid-phase sintering that enables the low-temperature sintering of materials with high sintering temperature, is driven by the liquid phase""s capillary pressure which is exerted among fine particles of a solid phase. For liquid-phase sintering, the following requirements are required.
First, the base dielectric composition is required to include enough amount of a liquid phase to completely cover primary particles thereof, and have some solubility. Additionally, good wettability of the base dielectric composition in the liquid phase is required. Above all, liquid-phase sintering requires the formation of a liquid phase. In this regard, the additives must react with the base dielectric composition to form a liquid phase. Further, the glass frit must have a suitable softening temperature (Ts). Another requirement is that the liquid phase formed has low viscosity as to flow over all particles, thereby uniformly wetting the base composition.
In addition, smaller primary particles incur larger capillary pressures and thus, show larger driving forces for the compaction. Also, the distribution of the liquid phase among the primary particles is an important factor affecting the densification.
Accordingly, the base dielectric composition is mixed with the glass frit and CuO and the mixture is thermally treated at a temperature somewhat higher than the Ts of the glass frit to form a liquid phase which is uniformly distributed over the base dielectric composition, followed by sintering, in accordance with the present invention. As a result, excellent densification can be obtained in the dielectric composition of the present invention. Therefore, it is important to define the molar ratio among the constituent compounds of the base dielectric composition, as well as the composition and amounts of the additives capable of forming a liquid phase by reaction with the base composition, that is, the amount of CuO and the composition and amount of the glass frit.
Useful in a base composition in the present invention is the composition comprising BaO in an amount of 10.0-20.0 mol %, Nd2O3 and Sm2O3 in an amount of 7.0-20.0 mol %, Bi2O3 in an amount of 0.5-5.0 mol %, and TiO2 in an amount of 60.0-80.0 mol % with the proviso that the total mol % of individual components is 100.
With the content of any component being out of the range therefor, the base dielectric composition exhibits too low a dielectric constant or too high a temperature coefficient of resonant frequency to use in practice. In detail, when BaO is used in an amount less than 10.0 mol %, TiO2 becomes more abundant or Nd2O3xe2x80x94TiO2 compounds, which are low in dielectric constant, are formed to increase the temperature coefficient of resonant frequency or to reduce the dielectric constant. On the other hand, at over 20 mol % of BaO, BaOxe2x80x94TiO2 compounds low in dielectric constant are formed to drive down the dielectric constant of the base composition. Thus, the content of BaO is preferably in the range of 10 to 20 mol %.
Also, in connection with the amount of BaO, amounts of Nd2O3 (Sm2O3) and TiO2 must fall within their respective above-determined ranges to provide high dielectric constant and stable TCF for the composition.
When Nd2O3 and Sm2O3 amount to more than 20 mol % in sum, an Nd2O3 (Sm2O3)xe2x80x94TiO2 phase low in dielectric constant becomes abundant, leading to a reduction in dielectric constant and Q value.
Serving to keep the dielectric constant high and to control TCF, especially to stabilize the phase, Bi2O3 is indispensable for the composition. Its amount is limited within 5.0 mol %: otherwise, the composition cannot be used as a dielectric material owing to its drastic decrease in Q value.
Useful in the present invention is a ZnOxe2x80x94B2O3xe2x80x94SiO2xe2x80x94PbO based glass frit. Its amount is preferably on the order of 1.0-10.0 wt %. Preferably, it comprises ZnO in an amount of 30-70 wt %, B2O3 in an amount of 5-30 wt %, SiO2 in an amount of 5-40 wt %, and PbO in an amount of 2-40 wt %.
B2O3 lowers the viscosity of the glass and accelerates the sintering of the dielectric ceramic composition of the present invention. Where B2O3 is used in an amount lower than 5 wt. %, the dielectric ceramic composition is likely to not be sintered at lower than 900xc2x0 C. With more than 30 wt % of B2O3, the dielectric ceramic composition has poor moisture resistance. Thus, its amount is preferably in the range of 5-30 wt. % in the glass frit.
More than 40 wt % of SiO2 results in an excessive increase in the softening temperature of the glass frit which therefore cannot act as a sintering aid. When SiO2 is present in an amount less than 5 wt %, its effect is not obtained. That is, a preferable amount of SiO2 falls within the range of 5-40 wt. %.
With less than 2 wt % of PbO, the glass frit has too high a softening temperature (Ts), making no contribution to the sintering of the dielectric ceramic composition. On the other hand, more than 40 wt. % of PbO lowers the Ts of the glass frit to improve the sintering of the composition, but has the problem of decreasing Q value. Considering these facts, the amount of PbO in the glass frit is defined in the range of 2-40 wt %.
It is preferred that ZnO is used in an amount of 30-70 wt %. Excessive amounts of ZnO lead to an increase in the softening temperature of the glass frit, making the low temperature firing impossible.
In accordance with the present invention, CuO is used in the dielectric ceramic composition of the present invention. For improving the sinterability and controlling dielectric properties, CuO is preferably added in an amount of 1.0-10.0 wt %. CuO plays a main role in liquid-phase sintering while the glass frit aids the completion of the sintering.
Supplemented with the above-defined amounts of the glass frit and CuO, the dielectric ceramic composition of the present invention can be sintered at less than 900xc2x0 C. and shows a dielectric constant of 50 or higher, a high Q value, and a TCF of xc2x120 ppm/xc2x0 C. or less.
Below, a description will be given of the preparation of the dielectric ceramic composition of the present invention.
The starting materials BaO, Nd2O3, Sm2O3, Bi2O3, and TiO2, each with a purity of 99.0% or higher, are weighed according to a desired composition of x BaO-y1 Nd2O3-y2 Sm2O3-w Bi2O3-z TiO2, and admixed in a wet manner. In this regard, the wet mixing is carried out by milling the starting materials in deionized water for about 16 hours with the aid of 3"PHgr" zirconia balls in a rod mill. The slurry thus obtained is dried and calcined. Preferably, the calcination is carried out at 1,100-1,150xc2x0 C. for about 2-3 hours at the heating rate of 5xc2x0 C./min. When the calcination temperature is too low, intermediate phases rather than complete phases are formed, giving rise to an increase in shrinkage. When the calcination is carried out at too high temperatures, on the other hand, the particles become too coarse to pulverize later.
After being weighed according to a desired composition, the glass frit components are melted at 1,200-1,400xc2x0 C., quenched in water, and dry-pulverized. Then, the coarse particles are finely pulverized into powder with a size of 0.5xcx9c1.0 xcexcm in ethyl alcohol because too large particle sizes results in a nonhomogenous mixture.
The base dielectric ceramic composition is admixed with the glass frit powder composition, together with appropriate amounts of CuO, in a batch. This admixing is preferably carried out for 16 hours.
Following drying, the powder thus obtained was thermally treated at 600-700xc2x0 C. preferably for 2-3 hours. The thermal treatment temperature is somewhat higher than the softening temperature (Ts) of the glass frit, so that a liquid phase of the glass frit is formed and coated uniformly over the base composition particles, thereby improving the reactivity and uniformity. By these reasons the sinterability of the dielectric ceramic composition can be improved.
Next, the thermally treated powder is further broken down into a desired particle size, and molded to a desired form such as a disc or a sheet.
Afterwards, electrodes are formed in the molded disc or sheet and co-fired at less than 900xc2x0 C. to produce a desired device.
Having generally described this invention, an improved understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.